Method and apparatus for determining depleted capacity of a battery

ABSTRACT

In a method and apparatus for determining the depleted capacity of a CF x  type battery used in an implantable medical device, average values of battery voltage and battery current drawn from the battery are measured during a measurement time, the length of which exceeds a battery voltage recovery time after a load change, and wherein the actual depleted capacity of the battery is determined by predetermined relations between combinations of the average values of voltage and current and depleted battery capacity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus fordetermining depleted capacity of a battery of CF_(x) type used in animplantable medical device.

2. Description of the Prior Art

So called CF_(x) (carbon monoFluoride) batteries offer the possibilityto use fast microprocessors in implantable medical devices likepacemakers, since this type of battery has the capability of deliveringcurrent pulses in the milliampere range required by most suitablemicroprocessors. Further, there is a growing interest in multi-chamberpacing and also in high rate pacing for arrhytmia suppression andtermination which also increases the need of the battery to deliverhigher battery current. Future products will require high speed and longrange telemetry, which also requires higher battery current.

However, the determination of the state of discharge or remainingcapacity of this kind of battery currently causes considerabledifficulties, since there is no single electrical quantity which is wellcorrelated to remaining usable battery capacity.

The battery voltage exhibits very long time constants after load changesand as a consequence there is no useful relation between theinstantaneous battery voltage and the state of discharge or remainingbattery capacity unless the battery load is constant. Measuring thebattery impedance is not useful either for this purpose, since it doesnot provide useful data during the whole discharge period but only inthe latter part of the battery lifetime. Thus, known conventionalmethods of determining the remaining capacity of batteries used inimplantable medical devices cannot be used for CF_(x) type of batteries.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a new technique fordetermining the status of CF_(x) type batteries, when used inimplantable medical devices, especially implantable heart stimulators.

The above object is achieved in accordance with the principles of thepresent invention in a method and apparatus for determining the depletedcapacity of a CF_(x) type battery used in an implantable medical device,wherein average values of battery voltage and battery current drawn fromthe battery are measured during a measurement time, the length of whichexceeds a battery voltage recovery time after a load change, and whereinthe actual depleted capacity of the battery is determined bypredetermined relations between combinations of the average values ofvoltage and current and depleted battery capacity.

Thus if the battery voltage and the current drawn for the battery areaveraged over a sufficiently long periods these average values can beused for determining the remaining capacity of the battery. The voltageand current are averaged over a measurement time exceeding the length ofa battery voltage recovery time after a load change, the measurementtime exceeding the recovery time preferably by a predetermined factorbetween 5 and 10. The recovery time can be defined as the time neededfor the battery voltage to reach a certain percentage, e.g. 90%, of itssteady state level.

In another embodiment of the method according to the invention, theaverage values of voltage and current are entered into a predeterminedlook-up table providing depleted battery capacity for different averagevoltage and current combinations. In practice such a look-up table isavailable from e.g. the battery manufacturer Wilson Greatbatch based onconstant current data. Experiments have, however, shown that if thebattery voltage and current are averaged over a sufficiently longmeasurement time combinations of the average voltage and average currentvalues can be used for obtaining reliable values of depleted batterycapacity from such a table.

In other embodiments of the method according to the invention depletedbattery capacity is also determined by time integrating the totalcurrent drawn from the battery. This technique for determining depletedbattery capacity is per se previously known when applied to other typesof batteries for implantable medical devices, see e.g. U.S. Pat. No.5,769,873. According to the invention an alarm is preferably triggeredif the difference between depleted battery capacities, determined frommeasured average values of battery voltage and current and determined bytime integration of the current drawn from the battery, respectively,exceeds a predetermined threshold value. The triggering of the alarmthen indicates that the depleted battery capacity has to be furtherconsidered or investigated.

In a further embodiment of the apparatus according to the invention theaveraging unit is adapted to determine the average values by samplingand integrating battery voltage and current during the measurement time.As discussed above the measurement time is in practice comparativelylong, e.g. 24 h, and the sampling frequency is chosen high enough to getgood accuracy of the average values, e.g. a sampling frequency of 256Hz. With the use of an optional filter in front of an analog to digitalconverter, the sampling frequency can be reduced, e.g. to the range 0.1to 1 Hz.

In further embodiments of an apparatus according to the invention animpedance measurement unit is provided to measure the internal batteryimpedance when depleted battery capacity reaches a predeterminedthreshold value and a second determining unit is provided to thereafterdetermine depleted battery capacity from the measured internalimpedance. Internal impedance measurements give reliable values of thedepleted battery capacity only in the fatter part of the batterylifetime. A first triggering unit is therefore preferably provided totrigger the impedance measurement unit when depleted battery capacityreaches the predetermined threshold value, determined from measuredaverage values of battery voltage and current as described above.

In another embodiment of the apparatus according to the invention asecond triggering unit triggers an alarm if the difference betweendepleted battery capacities, determined from measured average values ofbattery voltage and current, and determined by time integration of thecurrent drawn from the battery or determined from the measured internalimpedance, respectively, exceeds a predetermined threshold value. Thus,if there are discrepancies in the depleted battery capacities determinedby the different methods, this is indicated to the patient and/or thephysician so that further investigations can be made. In this wayimproved security with reference to the battery status is obtained.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of battery voltage and discharge capacity versus timeobtained from measurements on a CF_(x) type battery.

FIG. 2 is a block diagram of an embodiment of the apparatus according tothe invention.

FIG. 3 illustrates an example of a look-up table suitable for use fordetermining depleted battery capacity in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a plot of battery voltage and discharged capacity versus timeobtained from measurements on a CFx type battery. The battery wassubjected to different load patterns in a 17 weeks test sequence. Moreprecisely FIG. 1 shows the results obtained for four different loadpatterns simulating various types of pacemaker loads.

Pattern 1 includes 3 hours of 10 μA load followed by 9 hours of 5 μAload, repeated 14 times, which gives a total time of 7 days. Thispattern simulates a typical low current sequence with 3 hours of loadthreshold (Autocapture) single chamber pacing, followed by 9 hours ofinhibition.

Pattern 2 includes a fixed load of 6.25 μA during 7 days, representingthe average load of pattern 1.

Pattern 3 includes 3 hours of 100 μA load followed by 9 hours of 5 μAload, repeated 14 times, which gives a total time of 7 days. Thissimulates 3 hours of high threshold, multiple chambers pacing, followedby 9 hours of inhibition.

Pattern 4 includes a fixed load of 28.75 μA during 7 days, representingthe average load of the pattern 3.

Between each week of loads simulating typical pacemaker loads accordingto patterns 1–4 above one week follows with a heavy load ofapproximately 900 μA in order to discharge the battery within areasonably short time. In FIG. 1 such cycles are shown repeated 8 times.In FIG. 1 total depleted battery capacity is also showed as a functionof time.

In FIG. 1 can be seen that the dynamic impedance is high in thebeginning of the battery lifetime and then successively decreases. Itcan also be seen that the recovery time increases with the depletion ofthe battery. Thus at the time of about 60 days in FIG. 1 steady state isreached after a period of heavy load within a few days, whereas at time100 days steady state is hardly reached within one week.

In FIG. 2 an embodiment is shown of the apparatus according to theinvention implemented in a pacemaker.

A shunt resistor Rs typically of 100 Ohm is connected to the battery 2of CF_(x) type to be tested. This resistor Rs converts the current fromthe battery 2 to a voltage. The current drain from the battery 2consists of the internal housekeeping current and the current used fortherapeutic treatment, i.e. pacing pulses.

A voltage controlled oscillator (VCO) 6 converts the voltage across theresistor Rs to a pulse train with a frequency, which is proportional tothe voltage.

The counter 8 counts the pulse train pulses from the VCO 6. The count isread by the microprocessor 10 every 24 hours. Thereafter the counter 8is reset by the microprocessor 10 and starts counting for another 24hours period.

A stabilizing capacitor Cs typically of 47 μF is used for stabilizingthe supply voltage during varying battery current loads.

An analog to digital converter (ADC) 12, preceded by a RC-filter 14,converts the battery voltage to a digital word. The microprocessor 10controls the counter 8, reads the ADC 12 and calculates remainingcapacity of the battery 2 as will be further explained in the following.

The device circuitry 4 represents the complete normal circuitry of thepacemaker.

The average battery voltage is determined over a period of 24 hours. Thebattery voltage is sampled by the ADC 12. The 24 hours average voltageis calculated by the microprocessor 10 by calculating the sum of allsampled digital values during 24 hours and then dividing this sum by thenumber of samples. The voltage is sampled with such a high frequencythat good accuracy of the true average value is achieved, e.g. asampling frequency of 1 Hz.

The average battery current is also calculated over a period of 24hours. The current from the battery 2 is measured by measuring thevoltage across the resistor Rs. The measured voltage is supplied to theVCO 6, which is generating a pulse train with a frequency proportionalto the measured voltage, and consequently proportional to the current.This digital signal with a varying frequency is supplied to the counter8. The counter value is read every 24 hours. The counter 8 is thenimmediately reset to be ready for counting during the following 24 hoursperiod.

In the embodiment shown in FIG. 2 a VCO 6 is used for currentmeasurements and an ADC 12 is used for voltage measurements. Asalternatives either VCOs or ADCs can be used for both current andvoltage measurements. As another alternative the microprocessor can bereplaced by hard-wired logic.

The 24 hours average voltage and current values are entered into alookup table as shown in FIG. 3 to obtain remaining battery capacity.

The table in FIG. 3 is an example based on constant load current dataavailable from the battery manufacturer Wilson Greatbatch. However,experiments have shown that corresponding tables are valid for variableloads when using voltage and current average values determined asdescribed above.

The table in FIG. 3 is used as follows. The average current for the last24 hours is determined to e.g. 30 μA. The corresponding average voltagehas been determined to e.g. 2.940 V. The 30 μA row is then followed inthe table until the measured average voltage of 2.940 V is reached. Thiscolumn in the table is followed to the top of the table where thedepleted capacity can be read to 0.6 Ah. Interpolation is used todetermine a value for the depleted capacity when one or both of theaverage voltage and the average load current values are in between thevalues in the table.

The look-up table is preferably stored in the memory of themicroprocessor 10 and the described procedure is executed in automatedfashion. The invention can then be used as a new advantageous RRT(Recommended Replacement Time) indicator for CFx type batteries.

Although modifications and changes may be suggested by those skilled inthe art, it is the invention of the inventors to embody within thepatent warranted heron all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

1. A method for determining depleted capacity of a CF_(x) type batteryin an implantable medical device, comprising the steps of: measuring anaverage value of battery voltage and an average value of battery currentdrawn from a CF_(x) type battery during a measurement time having alength exceeding a battery voltage recovery time of said CF_(x) typebattery after a load change; and determining actual depleted capacity ofsaid CF_(x) type battery from predetermined relationships betweencombinations of said average values of voltage and current and values ofdepleted capacity.
 2. A method as claimed in claim 1 comprisingmeasuring said average voltage and average current during a measurementtime which exceeds said battery recovery time by a predetermined factor.3. A method as claimed in claim 2 wherein said factor is between 5 and10.
 4. A method as claimed in claim 1 comprising measuring said averagevoltage and said average current from a CF_(x) type battery having abattery recovery time in a range between 3 and 50 hours.
 5. A method asclaimed in claim 1 comprising measuring said average voltage and saidaverage current for a measurement time exceeding 24 hours.
 6. A methodas claimed in claim 1 wherein the step of determining said actualdepleted capacity comprises storing predetermined combinations ofaverage voltage and average current, with respect to depleted batterycapacity, in a look-up table, and consulting said look-up table todetermine a depleted battery capacity therein corresponding to saidcombinations of said measured average voltage and said measured averagecurrent.
 7. A method as claimed in claim 1 further comprisingintegrating current drawn from said CF_(x) type battery over time,thereby obtaining an integrated current value, and also determining saidactual depleted battery capacity from a relationship between depletedbattery capacity and said integrated current value.
 8. A method asclaimed in claim 7 comprising triggering an alarm if a differencebetween the actual depleted battery capacity determined from saidcombination of average battery voltage and average battery current, andthe actual depleted battery capacity determined from said integratedcurrent value, exceeds a predetermined threshold value.
 9. An apparatusfor determining depleted capacity of a CF_(x) type battery in animplantable medical device, comprising: an averaging unit adapted forconnection to a CF_(x) type battery for measuring an average value ofvoltage drawn from said CF_(x) type battery and an average value ofcurrent drawn from said CF_(x) type battery during a measurement timehaving a length exceeding a battery voltage recovery time of said CF_(x)type battery after a load change; and a determining unit connected tosaid averaging unit for determining actual depleted battery capacity ofsaid CF_(x) type battery from predetermined relationships betweencombinations of said average values of voltage and current and depletedbattery capacity.
 10. An apparatus as claimed in claim 9 wherein saiddetermining unit includes a look-up table wherein predeterminedrelationships between combinations of average battery voltage andaverage battery current and depleted battery capacity are stored, andwherein said determining unit consults said look-up table to determine adepleted battery capacity corresponding to said combinations of themeasured average voltage and the measured average current.
 11. Anapparatus as claimed in claim 9 wherein said averaging unit determinessaid average value of battery voltage by sampling an integrating saidbattery voltage during said measurement time, and determines saidaverage value of battery current by sampling and integrating saidbattery current during said measurement time.
 12. An apparatus asclaimed in claim 9 wherein said determining unit includes an integratingunit which integrates a total current drawn from said CF_(x) typebattery during said measurement time, to determine a total currentdepleted from said battery, and a second determining unit which alsodetermine said actual depleted battery capacity from a relationship tosaid total current.
 13. An apparatus as claimed in claim 12 comprising atriggering unit which triggers an alarm if a difference between saidactual depleted battery capacity determined from said combinations ofaverage values of voltage and current, and said actual depleted batterycapacity determined from said total current, exceeds a predeterminedthreshold value.
 14. An apparatus as claimed in claim 9 furthercomprising an impedance measurement unit which measures internal batteryimpedance of said CF_(x) type battery when said actual depleted batterycapacity reaches a predetermined threshold value determined from saidmeasured average values of battery voltage and battery current, andfurther comprising a second determining unit which thereafter determinessaid actual depleted battery capacity from a relationship betweendepleted battery capacity and said measured internal impedance.
 15. Anapparatus as claimed in claim 14 further comprising a triggering unitwhich triggers an alarm if a difference between said actual depletedbattery capacity determined from said combination of average values ofbattery voltage and battery current, and said actual depleted batterycapacity measured integral impedance exceeds a predetermined thresholdvalue.